A large number of designs of inductive and transformer-type linear displacement devices (transducers) are known in the present state of the art, that are widely used in different instruments and measuring information systems.
Most of the known linear displacement transducers are characterized by the presence of an error in the results of measurements, so-called additional error, caused by external factors. Many of the improvements in the designs of the known transducers are directed to increasing the accuracy of measurements by minimizing the additional error; nevertheless, this problem still remains urgent.
Known in the art is a device for measuring displacements, belonging to the category of transformer-type transducers (U.S. Pat. No. 5,010,298), which comprises a variable-inductance winding consisting of two coils arranged on a core made from a ferromagnetic material. The coils abut each other in an axial direction and are encompassed by a metallic screen, and the primary detector circuit is connected with the winding.
The overall dimensions of the known transducer are sufficiently large, because its length is approximately twice the displacement range, this being of crucial importance when the measuring ranges are large. Due to the large overall dimensions of the transducer its parts are under different conditions, and this leads to the appearance of an additional error caused by temperature and moisture gradients and other external factors.
Transducers for measuring linear displacements are also known, described in U.S. Pat. Nos. 5,210,490 and 5,216,364). Each of these transducers has two coaxial cores on which transformer windings are arranged. In the first version the primary and secondary transformer windings abut each other, and a movable member moves along them; in the second version the windings are arranged on the corresponding cores and the movable member moves in a gap between the cores. An electric circuit whose output signal bears information on the displacement of an object is connected to the windings.
The output voltage for these devices is found from the formula ##EQU1##
where .omega. and U are the frequency and amplitude of the supply voltage, Z.sub.1 is the impedance of the primary winding, and M(x) is the mutual inductance of the primary and secondary windings, dependent on the position x of the movable member. The value M(x) depends on the inductance of the windings, i.e., on the number of turns, the material and configuration of the magnetic system constituted by the cores and the movable member. The value Z depends on the inductance and active resistance, which is much more dependent on the ambient conditions (first of all, on the temperature) than the inductance is. Therefore, the values M(x) and Z vary to a different extent under the influence of external factors, and this leads to errors in the results of measurements.
Furthermore, in the embodiment of the device with the coaxial arrangement of the windings an additional error may originate because of the conditions being not the same along the transverse section, compared with the embodiment with the longitudinal arrangement of the windings, wherein a temperature gradient along the length may take place.
The technical solutions presented hereinbelow belong to a different group of transducers, namely, to inductive transducers.
For instance, an inductive linear displacement transducer is known (U.S. Pat. No. 4,954,776), comprising a winding arranged on a ferromagnetic core, a source of a-c voltage with a prescribed frequency, a movable member, and a thermosensitive measuring winding connected with the winding.
In the US Patent cited above the temperature effect is compensated for with respect to the resistance of the measuring winding, whereas measurements are effected in terms of the output voltage active component proportional to losses in the movable member, caused by eddy currents. Temperature variations lead to changes of the electromagnetic parameters (magnetic permeability, electrical conductivity) of the core and, hence, to changes in the value of losses which remains not compensated for.
Those components of the device which take part in the process of thermocompensation are under different conditions, and this leads to inadequate error compensation. The electrical compensation circuit is rather complicated.
Devices sufficiently close to the proposed invention are those for measuring linear displacements, based on the classical inductive divider circuit. However, there arises a problem characteristic of this type of transducers. The additional winding, which partially compensates for the additional error, must be found under the same physical conditions as the measuring winding, i.e., it must be disposed near the measuring winding all over the length thereof. But then, with the adopted manner of winding the turns, the additional winding will experience the influence of eddy currents of the movable member, and this will make the interpretation of the output signal ambiguous. In other words, it will be impossible to reveal the cause of variation of the additional winding impedance--whether it stems from the position of the movable member or from a change in the ambient conditions, this being equivalent to the appearance of a measurement error.
An inductive position transmitter is known (U.S. Pat. No. 5,331,277), belonging to the category of inductive dividers, which comprises an inductance coil with a constant and variable resistance that are interconnected and inserted between an a-c voltage source and ground, making-up a divider which shapes an output signal that varies in accordance with a prescribed relation.
The output voltage in the known device is determined from the formula ##EQU2##
where V is the power supply voltage, Z.sub.1 and Z.sub.2 are the impedances of the windings with the variable and constant inductance, respectively. These windings differ in extension, because the first is distributed along the length and the second is concentrated within a small area. This causes dissimilar variation of their impedances under the effect of varying ambient conditions and leads to errors in the output signal value. The winding with the variable inductance consists of several sections, and the number of turns is chosen in accordance with a definite law. Said sections are distributed according to length, and the active-to-reactive resistance ratio in them is different. As a result, the impedances of the sections vary to a different extent, and this leads to violation of the prescribed law of the winding inductance variation, and, hence, to an error.
The prior art nearest to the proposed invention is a device for measuring linear displacements of induction type (D. I. Ageikin et al., "Control and Regulation Transducers", Mashinostroenie, Moscow, 1965, p. 126 (in Russian), comprising a primary detector and a measuring amplifier, the primary detector comprising a central cylindrical core and an external cylindrical core arranged coaxially, a movable member mounted coaxially thereto, and electrically interconnected measuring and additional windings, of which the first is disposed along the length of the central core so that its turns encompass the central core in a transverse direction, and the measuring amplifier is electrically connected with the measuring and additional windings.
In the known device the movable member is made as a metallic tube from a nonmagnetic material, which embraces the central core, the measuring winding is arranged on a frame in a gap between the central core and the external core, and the additional winding in the form of a throttle together with the measuring winding are inserted into a bridge circuit fed with an audio-frequency a-c voltage. The bridge circuit outputs are coupled to inputs of a tensometric amplifier. The device is intended mainly for measuring large linear displacements (up to 100 m).
The operation of the device is based on changes in the total resistance of the measuring winding as the electrically conducting movable member approaches it under the demagnetizing effect of eddy currents induced in the measuring winding.
The design of the device is such that the measuring winding and the additional winding are under different physical conditions. An appreciable additional error caused by the influence of external factors. With the measuring and additional windings inserted into a bridge circuit, constant voltage across the measuring winding cannot be ensured, and this leads to the appearance of an additional error in the output signal, namely, of the multiplicative error component which is understood as an error due to a change in the sensitivity of the primary detector. Variations in the ambient conditions bring about changes in the resistance of the measuring winding. This causes the appearance of an additive component which is understood as an error due to a change in the initial value of the output signal, corresponding to the initial position of the movable member.
The design of the primary detector in the known device limits its functional potentialities, complicating adaptation to the service conditions (for instance, mating of the movable member with the object of measurement). The arrangement of the measuring winding on a frame between the cores does not allow one to reduce the cross-sectional area of the primary detector.